FIB/SEM Study of Pitting and Intergranular Corrosion in an Al-Cu Alloy

Scanning electron microscopy (SEM) with energy dispersive x-ray spectroscopy (EDX) was used to study the formation of stable pits on an aluminum-copper casting alloy in artificial seawater solution at 10°C. The polished specimens were immersed for various times and the development of corrosion around intermetallic particles was monitored. Focused ion beam (FIB)/SEM was used to determine the subsurface microstructure at selected local attack sites within the corroded area. The earliest stages of attack started with pit initiation at the surface of intermetallic particles resulting in dealloying. Cu redistribution occurred during corrosion over the entire surface and its concentration differed from one site to another. FIB imaging revealed that localized corrosion was propagated where continuous particles were buried beneath the surface. Dealloying of the particle and matrix/particle interfacial attack produced a sponge-like remnant and cracks at the interface, respectively. Propagating away from the initiation sites, corrosion developed preferentially along the grain boundary network beneath the alloy surface.